Systems are known for the control of the linear speed of a web moving between a supply and a takeup reel where the rotational velocities of the reels are continuously changing during web motion due to the respectively increasing and decreasing radii of web material on the two reels. Such systems are employed, for example, in magnetic tape cassette transports for digital data recording and retrieval and of the type in which the tape reels are directly driven and in which a capstan drive is not employed to maintain linear tape speed. According to one known control technique, clock information is prerecorded along a timing track on the magnetic tape and during transport operation this information is read from the tape and employed in a servocontrol loop to maintain constant tape speed. The presence of a clock track on the tape materially detracts from the data storage capacity of the tape and requires prerecording of the tape.
According to another known speed control technique, the equation of motion is solved to derive an error signal for controlling the takeup motor speed in order to yield a constant linear tape speed. The tape or web speed in terms of the supply and takeup reel velocities is defined as follows: ##EQU1## where s is linear tape speed (centimeters per second), f.sub.A and f.sub.B are the respective reel rotation frequencies (revolutions per second) and R.sub.E and R.sub.F are the respective empty and full reel radii (centimeters). This equation must be met if absolute web speed control is to be achieved. Systems in which the foregoing equation is electronically implemented either directly or indirectly to provide speed control are shown, for example, in U.S. Pat. Nos. 2,837,705, 3,806,061, 2,469,706, 2,949,249 and 3,733,529. In each of the systems of the above-identified patents the speed of both the supply and takeup reels is measured to provide signals which are processed in a manner to effectively satisfy the above equation to provide a control signal to a drive motor to maintain constant linear web speed. The circuit implementations required to effectively implement this equation tend to be needlessly complex and expensive since the theoretical degree of control cannot be realized in practical engineering embodiment.
Another speed control technique is the subject of the United States Pat. No. 3,704,401, in which signals representing the speeds of the two reels are summed and the summed signal compared with a reference signal to provide an error signal for motor control. Although relatively simple in implementation, this technique exhibits a significant error of about .+-.6% for a 150 foot tape, and .+-.14% for a 300 foot tape in a standard Philips-type cassette.